It is known that ethylene can be oligomerized to form low molecular weight oils, intermediate weight waxes or semisolids or polymerized to form high molecular weight polymers using Ziegler-Natta catalysts, i.e., a transition metal compound in combination with an alkyl metal halide. Catalysts in which the transition metal is titanium in compounds such as titanium tetrachloride, and in which the alkyl metal halide is an aluminum compound, such as an ethyl aluminum chloride, have been successfully used to prepare a variety of olefin oligomers and polymers. However, it is difficult to obtain oligomers of ethylene and higher olefins such as propylene which are suitable for lubricating oils directly without the use of hydrogen in the reaction or the necessity of thermal cracking to remove waxy components.
The preparation of ethylene-propylene copolymer oils using Ziegler-Natta catalysts is disclosed in U.S. Pat. No. 3,676,521 which is incorporated herein by reference. The patent explains that a synthetic lubricant desirably has maximum pour point of 0.degree. F. (-18.degree. C.), a minimum viscosity index (VI) of 110 and a viscosity at 210.degree. F. (99.degree. C.) of 1 to 60 cs. The patent also discloses the difficulty of achieving these characteristics without a cracking step which is necessary unless high hydrogen pressure of 500 to 1000 psig is used to control molecular weight. The reaction is conducted in an inert solvent such as hexane, although the solvent can be omitted by using excess propylene as the solvent.
The oligomerization of ethylene alone with an alkyl aluminum halide-titanium halide catalyst, in the presence of inert solvent which includes at least 40 volume percent of a halogenated aromatic hydrocarbon and optionally a diluent such as heptane, benzene, or xylene is disclosed in U.S. Pat. No. 2,993,942. The patent discloses that ethylene is unique to the invention in that other olefins do not respond to give similar products.
In accordance with this invention mixed aliphatic-aromatic solvents are used to prepare ethylene-propylene oligomers having high VI and low pour point, with Ziegler-Natta catalysts under low hydrogen pressure or in the absence of hydrogen.